A well-known homogeneous catalyst for olefin polymerization is a metallocene compound. After the report on isotactic polymerization by W. Kaminsky et al., a technique for polymerizing an olefin by means of a metallocene compound (in particular, technique for polymerizing an α-olefin) has been variously modified and studied to further improve tacticity and polymerization activity (Non Patent Literature 1).
In the report by J. A. Ewen, which is one of such studies, polymerization of propylene in the presence of a specific catalyst enables production of polypropylene having a high tacticity in which the syndiotactic pentad content is greater than 0.7 (Non Patent Literature 2). In this case, such a specific catalyst is composed of an aluminoxane and a metallocene compound having a ligand in which a cyclopentadienyl group is cross-linked with a fluorenyl group by isopropylidene.
In an approach to modifying such a metallocene compound, a 2,7-di-tert-butylfluorenyl group replaces the fluorenyl group to improve the tacticity (Patent Literature 1). Other approaches have been reported; a 3,6-di-tert-butylfluorenyl group replaces the fluorenyl group to improve the tacticity (Patent Literature 2), and the bridging moiety at which a cyclopentadienyl group has been bonded to a fluorenyl group is modified to improve the tacticity (Patent Literatures 3 and 4).
In another report, dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl)(fluorenyl)zirconium dichloride in which a methyl group has been introduced also to the fifth position of the cyclopentadienyl ring enables production of an isotactic polypropylene having a higher molecular weight as compared with dimethylmethylene(3-tert-butylcyclopentadienyl)(fluorenyl)zirconium dichloride (Patent Literature 5).
Furthermore, di(p-chlorophenyl)methylene(cyclopentadienyl)(2,7-diphenyl-3,6-di-tert-butylfluorenyl)zirconium dichloride has been also reported as a metallocene compound which enables polymerization at high temperature and which enables production of a high-molecular-weight polymer (Patent Literature 6).
Although the modifications of a metallocene compound have enabled production of an olefin polymer having high melting point and high molecular weight, the productivity of such an olefin polymer has been still insufficient in industrial production processes. In particular, in the case where the metallocene compounds are each used in the form of a solution in a hydrocarbon solvent, the solubility thereof is not high; hence, a large amount of solvents need to be used. In addition, using a large amount of solvents decreases the concentration of the catalyst in a catalyst solution containing the metallocene compound with the result that poisoning and deactivation leads to a reduction in the productivity. Hence, efficient production of an olefin polymer having high melting point and high molecular weight has been demanded.